I hate to correct you there, but some characters can't use some vehicles. It seems that the opposing team in a mission can't use the strongest vehicle of the enemy (for example: Rebels can't use AT-AT's on Hoth, Droids can't use Republic AT-TE's on Geonosis, etc).

The droidika is definatly one of the strongest player classes in the game, even thought it has many weeknesses such as:
1. very easy to be killed when rolling.
2. very slow when walking.
3. cannot side-step to avoid the obvious.
4. slow to turn.
5. cannot use vehicles because it is one.

but other than that if u know how to use it it can be a very effective assault tool.